INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Summarized financial data
Operating revenues	$ 1,548.7	$ 1,590.2	$ 3,657.3	$ 3,967.6
Operating expenses	1,209.9	1,275.6	2,691.9	3,110.2
Other expense, net	42.9	63.6	126.9	121.0
Current assets	1,713.4		1,713.4		$ 2,093.6
Noncurrent assets	33,328.6		33,328.6		32,858.2
Total assets	35,042.0		35,042.0		34,951.8
Current liabilities	3,484.6		3,484.6		3,182.7
Other noncurrent liabilities	1,093.4		1,093.4		1,128.9
Total liabilities and equity	35,042.0		35,042.0		34,951.8
ATC
Summarized financial data
Operating revenues	203.1	182.2	389.9	359.9
Operating expenses	97.5	93.6	192.7	184.0
Other expense, net	25.4	28.6	53.9	57.4
Net income	80.2	$ 60.0	143.3	$ 118.5
Current assets	93.1		93.1		84.7
Noncurrent assets	5,334.1		5,334.1		5,244.2
Total assets	5,427.2		5,427.2		5,328.9
Current liabilities	434.9		434.9		502.6
Long-term debt	2,412.5		2,412.5		2,312.8
Other noncurrent liabilities	321.7		321.7		298.9
Shareholders' equity	2,258.1		2,258.1		2,214.6
Total liabilities and equity	$ 5,427.2		$ 5,427.2		$ 5,328.9